Coffee Roasting: What’s Coffee Roasting

Green coffee interior cell wall

Green coffee interior cell wall

In this blog, we’ll look at what coffee roasting is and the significant changes in the coffee bean caused by heat, turning the tasteless green coffee to an aromatic and flavourful roasted coffee that we all enjoy. If you haven’t read the blog about green coffee beans, please do so because it’s the continuation of the crop to cup topic.

Coffee roasting

In a nutshell, roasting is simply applying heat to the green coffee bean and stopping the process when the bean reaches the desired temperature and aroma. However, the process gets more complex when considering the temperature distribution, the rate of change of temperature with the respect of time and the bean structure and its shape.

Green coffee has a shape of half an ellipsoidal body with a porous and heterogeneous inner structure. Given this, the roasting process creates an endothermic and exothermic reaction that causes physical changes such as the swelling of the inner structure. Thus, the application of heat in coffee beans not only generates a temperature field but also creates inner pressure and redistribution of moisture.

The roasting process is be illustrated in the diagram below (Eggers, 1995)

heat transfer image

Drying

Heat is transferred to the beans through convection, conduction or radiation. It penetrates the surface of the beans then conducts to the core creating a temperature gradient. The inner structure of the beans is porous and full of moisture. When the temperature reaches its evaporation point in the bean’s core, it starts evaporating towards the centre of the bean. At this stage, the bean’s wall is still relatively firm and strong enough to contain the vapour, creating a pressurised environment causing the beans to expand. This is the first part of the roasting process.


Due to the endothermic nature of the bean, the evaporation of the bean’s moisture needs latent energy to slow down the temperature rise in the inner bean. The swelling and drying results in a strong decrease in heat conductivity within the section between the vaporisation front and the outer surface of the bean. This is a consequence of the temperature gradient becoming steeper in the dried region on the bean because of the enhanced resistance to heat transfer.

Browning

The Maillard reaction responsible for the browning occurs rapidly around 130 °C - 140 °C. From 160 °C, the formation of flavour compounds occurs, starting from the bean surface and moving towards the dry porous in the inner bean. This moving latent heat is exothermic.

First crack

The exothermic reaction generates gas (mainly carbon dioxide) within the cell structure and increases the pressure until the wall is weakened and cracks,  producing an audible popping noise that is known as the first crack. At this stage, the bean density reduces by almost 50% of its initial value.

Temperature Development

The roasting process is much faster on the surface of the bean compared to the core. Thus, understanding the temperature gradient helps to properly develop inner beans temperature to produce homogenous colour in the roasted bean.

Eggers (year) in his 10 minutes roasting experiment analyses the temperature difference between the surface, core, and the area between the core and the surface as depicted in the graph below.

temperature development graph

At the start of the roast, the surface and the core is at equilibrium (ambient temperature). As heat is applied, the surface temperature experiences a steep increase while the core lag behind. During this first period of 0 - 50 sec in the roast, the difference between the surface and the core temperature is approximately 70 °C, while the temperature between the core and the surface approaches 10 °C. The substantial temperature difference between the core and surface continues for the first 3 minutes of the roast, while the temperature between the core and the surface increases approaching surface temperature.

A small temperature drop occurs just after 3 minutes of roasting time. After this phenomenon, the inner bean temperature increases rapidly approaching surface temperature due to the build-up pressure until it reaches the vaporization temperature creates an endothermic flash reaction resulting in slight temperature drop (Eggers, year). Immediately the full exothermic reaction is taking place.

To sum up, heat triggers complex changes and chemical reactions in coffee beans turning the tasteless green coffee to delicious coffee that we enjoy. The effect of temperature on the coffee bean is depicted in the table below (Maier, year):

Bean temperature (°C)

Effect

20 - 130

Moisture evaporates and dries up the bean. Colour fades

130 - 140

First endothermic reaction. Yellowing in colour, swelling of the bean and starting the Maillard reaction. Roast gases are formed and begin to evaporate.

140 - 160

Complex series of endothermic and exothermic peaks. Beans colouring to light brown. A large increase in bean volume and micropores. Silverskins are removed. Bean is becoming very brittle. The surface of the bean starts cracking. Aroma formation starts.

160 - 190

Roast reactions move toward the inner, dry structure of the bean.

190 - 220

Small cracks inside the bean. Smoke and large volumes of carbon dioxide escape and leave the bean very porous. The typical roasted coffee flavours are formed.


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